In recent years, various radio communication devices including WLAN (Wireless Local Area Network) are mounted on a wide variety of apparatuses such as a personal computer, a peripheral equipment, a cellular phone, a game player, an STB (Set Top Box) and a CE (Consumer Electronics) apparatus such as a TV set, which come into use in various places including ordinary houses. Among possible reasons for widespread use of the devices, one is that although the device is a radio device emitting radio waves, no license is required for its use and it is relatively reasonable enough to be accessible to everyone.
With a device accessible to everyone, safety of a communication path between radio sections and access control for preventing use at will are crucial. Under these circumstances, the mechanism of encryption of a communication path between radio sections and the mechanism of authentication at the time of use are taken into consideration.
Although it is difficult to comprehend the contents of data communication because of encryption of data communication, the fact that data communication being executed cannot be hidden. There also exists a case where by comprehending an interval or timing at which a data frame is transmitted and received in data communication, and a data size, it is possible to presume by which application the communication is being executed. Assume, for example, that a data frame of approximately 300 bytes are interactively transmitted and received at an interval of several tens of times per second. In this case, it can be considered that voice call is being made.
In communication over a cable network, since data can be obtained only by a device physically connected to the network, it is highly unlikely that data is obtained by an unintended third party. With respect to a problem that devices on the same L2 layer are allowed to obtain communication data of other device, while data has been initially transmitted to all the devices on the same L2 layer by the use of a device called a shared hub, use of a device called a switching hub enables transmission of data only to a least necessary port. Accordingly, no further consideration is required for solving the problem.
Among representative devices using radio communication is a cellular phone. A radio communication system used for a cellular phone, however, requires a license for emitting radio waves and requires a special device not reasonable for transmitting and receiving radio waves, so that it is not easily accessible to everyone. No consideration is required for solving the problem as well.
In other words, there exists a problem that when a unit having a device using a radio communication system which allows such data communication conditions as described above to be seized is put into practical use in an ordinary house by the use of such a device accessible to everyone as WLAN, for example, seizing data communication conditions at site by an unintended third party leads to comprehending which kind of data communication being executed, or a problem of privacy that existence of a person himself/herself can be presumed by the existence/non-existence of data communication. In a case where such a radio communication device is used that is mounted with a radio communication system by which a MAC address inherently applied to a terminal is ordinarily transmitted and received without its encryption on a radio section as a WLAN, the problem becomes more conspicuous.
One example of related radio devices is disclosed in Literature 1 (Japanese Patent Laying-Open No. 2004-289875). Disclosed in Literature 1 is a cellular phone which requires no dedicated starting switch, is adaptable to an existing cellular phone and has a security function that makes release of pseudo calling operation difficult by others than an owner. With the cellular phone having a security function, an alarm sound stops only when an operation button is continuously pressed for a predetermined period of time, when it is pressed by a predetermined number of times or when a predetermined code number is entered, and it fails to stop even when other button than the operation button is pressed or when other number than a predetermined code number is entered, which allows only an owner of the cellular phone with the security function to release the alarm sound with ease.    Literature 1: Japanese Patent Laying-Open No. 2004-289875.
The radio communication system including the technique disclosed in Literature 1 has the following problems.
The first problem is that data transmission conditions in a radio terminal having a radio communication device cannot be hidden. The reason is that the radio communication system recited in Literature 1 is allowed to receive communication of other radio device communicating on the same channel than its own radio device with ease.
The second problem is that when data communication is enciphered, although it is highly unlikely to obtain contents of the data communication, it is impossible to prevent presumption of which application realizes the data communication based on a data communication flow. The reason is the same as that of the first problem.
The third problem is that non-existence of a radio device might be presumed based on a failure of observation of data communication. The reason, which is the same as that of the first problem, is that conversely when no data communication is observed, it is possible to presume that there fails to exist a radio device.
The fourth problem is that flexible generation and transmission of a pseudo data frame is impossible for making it difficult to specify actual data communication. The reason is that no consideration is given to a pseudo data frame in the radio communication system recited in Literature 1.